With the ever increasing crime rate, increased attention has been placed on means for preventing the commission thereof. Crimes such as burglary are usually committed during the nighttime under the cover of darkness. Police and other security personnel have a need for instrumentalities to detect the criminal, particularly when the criminal is some distance away, as might be snipers hidden on roof tops or intruders within large buildings or areas to be secured.
In recent years security personnel have had the aid of devices employing image intensifier tubes to assist them in detecting intruders. However, often the persons to be detected are hidden behind objects, in bushes or elsewhere, so as to make detection difficult if not impossible.
It has been proposed to use optical retroreflection principles to detect the presence of these intruders. However, devices using these principles to detect the intruders human eye or his vision aids such as binoculars or periscopes, will also detect reflected light from broken glass, tin cans and dew drops, as well as reflected light from those of a cat or other nocturnal animal, thereby making the false alarm rate intolerable.